Methods for forming multilayer horizontal NOR-type thin-film memory strings
Various methods overcome the limitations and achieve superior scaling by (i) replacing a single highly challenging high aspect ratio etch step with two or more etch steps of less challenging aspect ratios and which involve wider and more mechanically stable active strips, (ii) using dielectric pillars for support and to maintain structural stability during a high aspect ratio etch step and subsequent processing steps, or (iii) using multiple masking steps to provide two or more etch steps of less challenging aspect ratios and which involve wider and more mechanically stable active strips.
1. A method for high aspect ratio etching for a memory structure to be formed above a planar surface of a semiconductor substrate, comprising:
preparing a plurality of active layers (“multilayers”) over the planar surface, stacked one on top of another along a first direction substantially orthogonal to the planar surface, wherein each multi-layer comprising first and second layers of a first conductive material;
providing a plurality of dielectric pillars extending along the first direction through the multilayers; and
patterning and etching the multilayers along the first direction using a mask to create a set of trenches that divide the multi-layers into a group of multi-layer stacks, wherein each trench extends along a second direction substantially parallel to the surface of the substrate and wherein each dielectric pillar abuts two neighboring multi-layer stacks.
2. The method of claim 1 , wherein the first conductive material comprises a doped semiconductor material.
3. The method of claim 1 , further comprising filling the trenches with a dielectric material and replacing a portion of the dielectric material in each trench by a plurality of conductive columns, each conductive column extending along the first direction.
4. The method of claim 3 , wherein replacing a portion of the dielectric material in each trench comprises:
etching the dielectric material in the trench to provide a plurality of shafts that extends along the first direction;
depositing a data storage material conformally in each shaft; and
filling each shaft with a second conductive material to form the conductive columns.
5. The method of claim 4 , further comprising, prior to preparing the multi-layers, providing a plurality of conductors above the surface of the semiconductor substrate, each conductor extending along a third direction substantially orthogonal to each of the first and second directions.
6. The method of claim 5 , wherein each conductor comprises a doped semiconductor or a metal.
7. The method of claim 5 , further comprising providing a conductive path to enable each conductive column to be electrically connected to one of the conductors.
8. The method of claim 4 , further comprising providing a plurality of conductors above the multi-layer stacks, each conductor extending along a third direction substantially orthogonal to each of the first and second directions.
9. The method of claim 8 , further comprising providing conductive paths to enable each conductive column to be electrically connected to one of the conductors.
10. The method of claim 8 further comprising:
providing circuit elements in the substrate; and
electrically connecting selected ones of the conductive columns to corresponding ones of the circuit elements, each of the selected conductive columns being situated substantially above its corresponding circuit element.
11. The method of claim 10 , further comprising providing an isolation layer between the circuit elements and the conductive columns, wherein electrically connecting selected ones of the conductive columns to their corresponding circuit elements comprises creating conductive paths through the isolation layer.
12. The method of claim 1 , further comprising creating a staircase structure at one or more ends of each multi-layer stack, wherein each staircase structure has steps and wherein each step extends a corresponding one of the multi-layers of the multi-layer stack along the second direction.
13. The method of claim 12 , further comprising providing an electrically insulating material over the staircase structure and providing via connections along the first direction to electrically connect to the first layer or the second layer in the multi-layer of a step of the staircase structures.
14. The method of claim 1 , wherein each multi-layer further comprises a sacrificial layer between and in contact with the first and the second layers of the first conductive material, the method further comprising replacing a portion of the sacrificial layer by a semiconductor material.
15. The method of claim 1 , wherein each multi-layer further comprises a sacrificial layer adjacent the first layer of the first conductive material, the method further comprising replacing the sacrificial layer by a second conductive material.
16. The method of claim 15 , wherein the second conductive material comprises a metal.